DOE OSTI2014
The Risk-Informed Safety Margin Characterization (RISMC) pathway of the DOE’s Light Water Reactor Sustainability (LWRS) program focuses on advancing the state of the art in safety analysis and risk assessment to support decision-making on nuclear power plant operation well beyond the originally designed lifetime of the plants (i.e., beyond 60 years). Among the issues being addressed in RISMC is the significance of SSC aging and how confident we are about our understanding of its impact on the margin between the loads SSCs are expected to see during normal operation and accident conditions, and the SSC capacities (their ability to resist those loads) as the SSCs age. In this paper, a summary is provided of a case study that examines SSC aging from an environmental qualification (EQ) perspective. The case study illustrates how the state of knowledge regarding SSC margin can be characterized given the overall integrated plant design, and was developed to demonstrate a method for deciding on which cables to focus, which cables are not so important from an environmental qualification margin standpoint, and what plant design features or operating characteristics determine the role that environmental qualification plays in establishing a safety case on which decisions regarding margin can be made. The selection of cables for which demonstration of margin with respect to aging and environmental challenges uses a technique known as Prevention Analysis. Prevention Analysis is a Boolean method for optimal selection of SSCs (that is, those combinations of SSCs both necessary and sufficient to meet a predetermined selection criterion) in a manner that allows demonstration that plant-level safety can be demonstrated by the collection of selected SSCs alone. Choosing the set of SSCs that is necessary and sufficient to satisfy the safety objectives, and demonstrating that the safety objectives can be met effectively, determines where resources are best allocated to assure SSC performance margin. The paper describes the resulting component types that were selected by Prevention Analysis and identifies the accident sequence characteristics that cause these component types to be important from an EQ and aging perspective (and, hence, worthwhile evaluating the extent of safety margin). In addition, component types not selected as needing significant margin from an EQ and aging perspective are discussed and an engineering rationale is developed justifying the lack of need to apply resources to demonstrating margin for these component types. This rationale is in terms of design features of the plant and operating characteristics that make these component types less important from an EQ and aging perspective. While the case study focuses on EQ and aging of equipment and cables located inside the containment of this PWR, the prevention analysis method is demonstrated to be an effective technique for identification of minimal collections of components that would be effective in managing safety for a variety of issues associated with aging and long-term operation of the fleet of plants.
22 GENERAL STUDIES OF NUCLEAR REACTORS↗